The Hartmann technique can be used to characterise an optical wavefront. In previously known implementations of this technique exact knowledge of the distance between the beam-sampling element and the sensor is required to calculate accurately the local slope of the wavefront at the beam-sampling element. The Hartmann technique can also be used to estimate a characteristic of an optical system by comparing the shape of the wavefronts before and after the system and using the distance between one of the cardinal points or planes of the system and the beam-sampler or the sensor as appropriate.
However, the locations of the cardinal points or planes are often unknown and not readily measurable. Inaccurate knowledge of these locations introduces systematic errors in the distance between the cardinal points or planes of the system and the beam-sampler or sensor. This results in an inaccurate map of the optical characteristics of the system or inaccuracies in estimates of the values for optical characteristics.
A practical optic may approximate an ideal optic. Generally there are imperfections with the approximation across the optic. It is desirable to map the imperfections of an optic as indicated by the optical characteristics for the optic. A map may be used to design or check desired performance characteristics for the optic. However, as mentioned, there is typically an error range for the position of the cardinal points or planes which leads to a corresponding error range in the mapped characteristics.
Some previously known methods and techniques for determining the characteristics of an optical wavefront or system are not suited to automated testing. Others have been automated but are complex and require specialised and delicate equipment. Reference in this specification to a document is not to be taken as an admission that the disclosure therein constitutes common general knowledge in Australia.